The present invention relates to chromophores with very large two-photon absorption cross-sections.
The two-photon process, predicted theoretically in 1931 and observed experimentally in the 1960s, has received little consideration for practical application. The lack of the availability of dyes with sufficiently large cross-sections has made many practical applications appear unattainable. Recently, the synthesis of new dyes with increased cross-sections and large upconverted fluorescence has opened up a myriad of new applications. These new applications include two-photon upconverted lasing, two-photon optical power limiting, three-dimensional optical data storage, and photodynamic therapy. Another application which has, unlike the others, received a reasonable amount of attention is three-dimensional imaging using two-photon laser scanning confocal microscopy. Multi-photon microscopy appears to be of great value as an imaging technique for numerous biological systems as well as organic paints and coatings. This technique, like the others, has never reached its full potential due to the lack of dyes which exhibit high intensity upconverted fluorescence. A tremendous improvement in the depth of confocal microscopic imaging can be obtained when the two-photon peak occurs at or near 800 nm, a wavelength at which most organic and biological materials have large optical transparency. It follows that a major molecular design challenge is to increase the molecular two-photon cross-section without shifting the two-photon absorption peak away from 800 nm. The U. S. Air Force currently has a strong interest in the development of two-photon technology as both an imaging tool for the nondestructive evaluation (NDE) of aircraft paint and as a useful material for optical power limiting. It is evident that in order for two-photon technology to realize its full potential, major improvements will be necessary in the design and synthesis of more active dye molecules with the necessary solubility and photo-stability.
The probability of absorption of two long wavelength photons from the laser source simultaneously is dependent upon the two-photon cross-section of the dye molecule. In U.S. Pat. No. 5,770,737, Reinhardt et al, issued Jun. 23, 1998, there are described asymmetrical fluorene-containing two-photon chromophores of the formula: EQU D--Ar--A
wherein the Ar core is ##STR2## PA1 wherein R.sub.1 and R.sub.2 are alkyl groups having 8 to 12 carbon atoms, and wherein R.sub.1 and R.sub.2 are the same or different, wherein D is an electron donor moiety selected from the group consisting of ##STR3## PA1 and wherein A is an electron acceptor moiety selected from the group consisting of ##STR4## PA1 wherein Ar is selected from the group consisting of ##STR5## PA1 --Fl--, --Fl--C.sub.6 H.sub.4 --, --Fl--Fl--, and --Fl--Fl--Fl--, PA1 wherein Fl is a fluorene group of the formula: ##STR6## PA1 wherein R.sub.1 and R.sub.2 are alkyl groups having 2 to 20 carbon atoms, and wherein R.sub.1 and R.sub.2 are the same or different; PA1 wherein D is ##STR7## PA1 wherein Q is selected from the group consisting of --H, --OH and --O--C.sub.x H.sub.2x+1, wherein x has a value of 1 to 10; PA1 and wherein A is selected from the group consisting of ##STR8## PA1 wherein Q is a single bond or 1,4-phenylene, Ph is a phenyl group, n has a value of 1-3 and m has a value of 3-n, and wherein T is ##STR9## PA1 wherein R.sub.1 and R.sub.2 are alkyl groups having 1 to 20 carbon atoms, provided that when Q is a single bond, the value of n is 2 or 3. Preparation of these chromophores is described in the Examples which follow. PA1 wherein T is as defined above, Q is a single bond or 1,4-phenylene, Ph is a phenyl group, n has a value of 1-4 and m has a value of 4-n, and wherein G is a core unit selected from the group consisting of ##STR10## ##STR11## PA1 wherein T is as described previously, Q is a single bond or 1,4-phenylene, Ph is a phenyl group, n has a value of 1-6 and m has a value of 6-n, and wherein G is a core unit having the structure ##STR12##
The most active dyes described in U.S. Pat. No. 5,770,737 incorporate an easily polarizable olefinic double bond in the backbone of the molecule. This olefinic bond, although greatly increasing the two-photon absorption (TPA) cross-section of chromophores, has limited thermal and photochemical stability, thus reducing the range of its utility.
In U.S. Pat. No. 6,100,405, issued Aug. 8, 2000, to Reinhardt et al there are disclosed dyes with increased thermal and photochemical stability while maintaining the same level of two-photon activity. These asymmetrical two-photon chromophores have the formula: EQU D--Ar--A
wherein Z is selected from the group consisting of --O-- and --S--.
Accordingly, it is an object of the present invention to provide more active dye molecules with the necessary solubility and photo-stability.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.